We have been developing a radioactive waste package made of high-strength and ultra low-permeability concrete (HSULPC) for geological disposal of TRU wastes, which is expected to be much more impervious to water than conventional concrete. In this study, basic data for the HSULPC regarding its the impervious character and the thermodynamics during cement hydration were obtained through water permeability measurements using cold isostatic pressing (CIP) and adiabatic concrete hydration experiments, respectively. Then, a prediction tool to find concrete package construction conditions to avoid thermal cracking was developed, which could deal with coupled calculations of cement hydration, heat transfer, stress, and cracking. The developed tool was applied to HSULPC hydration on a small-scale cylindrical model to examine whether there was any effect on cracking which depended on the ratio of concrete cylinder thickness to its inner diameter. The results were compared to experiments. For concrete with a compressive strength of 200MPa, the water permeability coefficient was 4 x 10(sup 19) m/s. Dependences of activation energy and frequency factor on degree of cement hydration had a sharp peaking due to the nucleation rate-determining step, and a gradual increase region due to the diffusion rate-determining step. From analyses of the small-scale cylindrical model, dependences of the maximum principal stress on the radius were obtained. When the ratio of the concrete thickness to the heater diameter was around 1, the risk of cracking was predicted to be minimized. These numerical predictions from the developed tool were verified by experiments.
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